Appendix N Methods of urinary iodine analysis and quality control

N.1 Introduction

From the start of Year 6 of the NDNS RP a spot urine sample was collected for iodine analysis from participants aged 4 years and over from whom consent had been obtained. Information regarding the collection, processing and storage of spot urine samples is provided in appendix M.

N.2 Iodine

Spot urine iodine concentrations were determined by measuring the $^{127}$I isotopes using an inductively coupled plasma mass spectrometer (ICP-MS). Samples were introduced to the ICP-MS via a flow injection system combined with the Sea spray nebulizer and cyclonic spray chamber arrangement.

Urine samples and quality control (QC) materials were prepared in diluent which included Tellurium (Te) as internal standard. The iodine isotope signal was compared against the internal standard, enabling any signal fluctuation due to instrument drift to be accounted for.

Matrix matched external calibration standards were prepared in pooled human urine (collected at the Medical Research Council Elsie Widdowson Laboratory (MRC EWL)) for each analytical batch.

Prior to analysis the ICP-MS instrument was tuned for optimum signal sensitivity and minimum oxide species and doubly charged ion formation. Unknown samples, blanks, calibration standards and QCs were analysed in each batch and the signal data generated was converted to concentration data via the calibration plot.

N.2.1 Quality controls for Iodine

In order to establish quality assurance of each analytical batch and inter-batch variation across the year’s cohort as a whole, ClinChek Urine Control Lyophilised for Trace Elements Level 1 and 2 (Recipe Chemicals and National Diet and Nutrition Survey Years 1 to 9 of the Rolling Programme (2008/2009 – 2016/2017): Time trend and income analysis Instruments GmbH) QC samples were analysed in conjunction with the blanks, calibration standards and samples.

N.2.2 Inter-batch variability

Table N.1 summarises the measured concentration of iodine following analysis of the QC samples for each individual year of the NDNS RP. For Year 9 of the NDNS RP the mean measured concentration of the QC was within the target concentration range defined by the manufacturer and CV was ≤5% for this survey year, showing that there was acceptable analytical accuracy and precision for samples from Year 9 of the NDNS RP. The QC data for Years 6 to 8 are presented in Appendix N of the NDNS Years 5 and 6 (combined)(^1) and Years 7 and 8 (combined)(^2) reports respectively.

**Table N.1: QC analysis for NDNS RP Year 9**

| ClinChek urine | Level 1 | Level 2 | |----------------|---------|---------| | Lot number | 432 | | | Target Concentration /Range (µg/L) | 120 (89.9 – 150) | 497 (373 – 622) | | Mean Measured Concentration (µg/L) | 114 | 484 | | N (QC samples) | 62 | 62 | | SD | 3 | 13 | | %CV | 2 | 3 | N.2.3 External Quality Controls for Iodine in Urine

EWL participates in the Inter-laboratory Comparison Program for Ensuring the Quality of Urinary Iodine Procedures (EQUIP), operated by the US Centers for Disease Control and Prevention (CDC). Iodine analysis of urine samples gives values which are within the criteria defined in this multi-laboratory programme achieving a “Successful Participation Certificate” with a laboratory score greater than 80%.

Table N.2 shows the percentage bias relative to the target concentration in urinary iodine results from MRC EWL during Year 9 of the NDNS RP.

Table N.2: Summary of EQUIP bias assessment in NDNS RP Year 9

| Urine Iodine EQUIP | Year 9 | |--------------------|--------| | mean % bias from target values | -5.2 | | SD % bias | 3.2 | | Number of EQUIP samples | 8 |

1 National Diet and Nutrition Survey: Results from Years 5 and 6 (combined) of the Rolling Programme (2012/2013 – 2013/2014). [Internet]. Available from: www.gov.uk/government/statistics/ndns-results-from-years-5-and-6-combined. 2 National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015 – 2015/2016). [Internet]. Available from: www.gov.uk/government/statistics/ndns-results-from-years-7-and-8-combined.